The accurate prediction of motion in waves of a marine vehicle is essential to assess the maximum sea state vs. operational requirements. This is particularly true for small crafts as Autonomous Surface Vessels (ASV). We consider two different methods to prediction motions of the SWATH-ASV: an inviscid strip theory initially developed at MIT for catamarans and then adapted for SWATHs and a hybrid strip theory, based on the numerical solution of the radiation forces by an unsteady viscous, non-linear free surface flow solver. Motion predictions obtained by the viscous method are critically discussed against those obtained with the potential flow based strip theory. Effects of viscosity are analyzed by comparison of sectional added mass and damping calculated at different frequencies and for different sections, three dimensional hydrodynamic radiation forces and RAOs. A number of Important conclusions can be drawn from this study: influence of viscosity is definitely non negligible for SWATH types of vessels like the one presented; the hybrid strip theory method with fully non-linear, viscous free surface calculation of the radiation forces appears to be a very valuable tool to improve the accuracy of traditional strip theories, without the burden of long computational times requested by fully viscous time domain three dimensional simulations.